Slide 4:

Light source :

Light source Distribution of energy through spectrum is function of temperature. For Visible region- Tungsten filament lamp Use for region 350nm to 2000nm. Problem- Due to evaporation of tungsten life period decreases. It is overcome by using tungsten-halogen lamp. Halogen gas prevents evaporation of tungsten.

Slide 6:

For ultra violet region- Hydrogen discharge lamp consist of two electrode contain in deuterium filled silica envelop. gives continuous spectrum in region 185-380nm. above 380nm emission is not continuous. UV-Vis spectrophotometer have both deuterium & tungsten lamps. Selection of lamp is made by moving lamp mounting or mirror to cause the light fall on monochromator.

Slide 7:

Deuterium lamps:- Radiation emitted is 3-5 times more than the hydrogen discharge lamps. Xenon discharge lamp:- Xenon stored under pressure in 10-30 atmosphere. It possesses two tungsten electrode separated by 8 cm. Intensity of UV radiation more than hydrogen lamp. Mercury arc:- Mercury vapour filled under the pressure . Excitation of mercury atom by electric discharge

Slide 9:

Prisms- Prism bends the monochromatic light. Amount of deviation depends on wavelength. Quartz prism used in UV-region. Glass prism used in visible region spectrum. Function – They produce non linear dispersion. They are universal and versatile ,used to get desired wavelength.

Slide 10:

Fig.-Mechanism of prism working Fig.-mechanism of working of prism .

Slide 11:

Grating- Large number of equispaced lines on a glass blank coated with aluminum film. Blaze angle Normal surface vector Normal to groove face

Types of grating:

Types of grating Two types of gratings:- Transparent grating Refraction or Diffraction grating Transparent grating:- Grating is transparent & radiation enters through one side that passes through the grating & separation occurs at other surface.

Diffraction Grating:

13 Diffraction Grating Mechanism : The rays which are incident upon the gratings gets reinforced with the reflected rays and hence the resulting radiation has wavelength which is governed by the equation: m λ = b (sin i ± sin r) Where λ = wavelength of light produced b = grating spacing i = angle of incidence r = angle of reflection m = order (0,1,2,3 etc)

Slide 14:

Spectroscopy requires all materials in the beam path other than the analyte should be as transparent to the radiation as possible. The geometries of all components in the system should be such as to maximize the signal and minimize the scattered light. The material from which a sample cuvette is fabricated controls the optical window that can be used. Some typical materials are: Optical Glass - 335 - 2500 nm Special Optical Glass – 320 - 2500 nm Quartz (Infrared) – 220 - 3800 nm Quartz (Far-UV) – 170 - 2700 nm •Keep the cuvette clean. •Don’t clean with paper products. •Store dry. •Don’t get finger prints on them. •Store carefully and gently Sample cell (cuvette)

Detectors:

Detectors Three common types of detectors are used Barrier layer cells Photocell detector Photomultiplier Photo voltaic cells or barrier layer cells :- They are primarily used for measurement of radiation in visible region. Maximum sensitivity-550nm. It consist of flat Cu or Fe electrode on which semiconductor such as selenium is deposited. on the selenium a thin layer of silver or gold is sputtered over the surface.

Slide 16:

A barrier exist between the selenium & iron which prevents the electron flowing through iron. Therefore electrons are accumulated on the silver surface. These electrons are produced voltage. - terminal Silver surface selenium + terminal fig.-Barrier layer cell

Photovoltaic or Barrier layer cells:

Photovoltaic or Barrier layer cells MECHANISM: Electrons are transferred at the interface from the semiconductor to metal when the semiconductor is irradiated. The electric current produced is proportional to the radiance power of the incident beam and to the area of photosensitive being irradiated. 17

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18 Merits : At low level of illumination it produces photo current proportional to the radiant power received on it. It is relatively cheap and widely used in filter photometers and cheap spectrophotometers. Demerit s: Slow response. Fatigue effect . While using it there should be no fluctuations in power and low external electric resistance.

Slide 19:

Photocell detector:- It consist of high sensitive cathode in the form of a half cylinder of metal which is evacuated. Anode also present which fixed along the axis of the tube Photocell is more sensitive than photovoltaic cell. + - light Fig.- photocell detector

Photomultiplier Tubes:

20 Photomultiplier Tubes Mechanism: It has a number of photo emissive electrodes each charged at a successively higher potential and so arranged that the electrons ejected from the photocathode travel successively from one electrode to the next. In each step the photocurrent is increased by the secondary emission of electrons.

Photo multiplier tube:

Photo multiplier tube

Merits:

22 Merits It is highly sensitive to light. Best suited where weaker or low radiation is received.

Recorder:

Recorder Signal from detector received by the recording system The recording done by recorder pan.

UV INSTRUMENTS-TYPES:

24 UV INSTRUMENTS-TYPES Colorimeters Spectrophotometers

Double Beam Colorimeter:

25 Double Beam Colorimeter

UV- spectrophotometer:

UV- spectrophotometer Single beam spectrophotometer:- Double beam spectrophotometer:- Advantage of double beam spectrophotometer:- It is not necessary to continually replace the blank with the sample or to adjust the autozero. The ratio of the powers of the sample & reference is constantly obtained. It has rapid scanning over the wide wavelength region because of the above two factors.

INSTRUMENTAL FACTORS CAUSING DEVIATION FROM BEER’S LAW:

Slide 31:

Polychromatic Radiation:- Spectral Slit Width(S.S.W) is a measure of extent of monochromatic radiation. Monochromator allows only specific band of radiation to pass through it and absorbs all other radiation. Spectral slit width is a range of wavelength in nm coming out of a monochromator & passing through solution.

Slide 32:

Measurement Speed:- It is a time required to measure the entire spectrum. Stray light:- It is the light reaching to the light measuring device other than light coming from the sample When light comes out of a monochromator some of the light travels in another direction & this light may get reflected by dust or other particles & reach the measuring device called as Stray Light.